In the extrusion of tubes as in the blown film process it is desired to guide or stabilize the tube with external guide members. Where cooling air is introduced to the tube, it is also desired to sense the tube size and control the volume of air within it. This has been achieved in prior art structures by curved guide members adjusted in slides in the radial direction of the tube but such arrangement has been cumbersome and has limited the number of guides employed about the circle, limiting the degree of engagement, especially for the larger sizes of tubes. The supporting structure of such arrangement also obstructs the operator's access to the space above the die for maintenance or startup of the extrusion process.
The prior art also suggests the use of widely spaced, small pads supported on the end of pivoted supports. In such arrangement the limited contact with the periphery of the tube can similarly detrimentally affect the degree to which the tube is guided or stabilized and can also obstruct operator's access to the die.
Objects of the invention are to provide an improved guide assembly for extruded tubes which according to various aspects of the invention overcomes the difficulties mentioned above and in particular enables a wide range of adjustment of tube diameter, good control of the circular shape of the tube over the range of adjustment, accurate regulation of the air volume within the tube during internal air cooling and general improved operation of the assembly.
The invention concerns a guide assembly of the type comprising a set of tube-engaging guide members arranged in a tube-encompassing circle, each guide member carried at the end of an elongated support arm, each arm pivoted at its opposite end to a supporting frame with the line between the connections forming a substantial angle with the radius of the tube, so that rotation of each arm about its pivot adjusts the spacing of the guide member from the axis of the tube. According to one aspect of the invention, each guide member is pivoted to its support arm and a compensatory rotation means dependent upon rotation of the arm rotates the guide member about its pivot opposite to rotation of the arm, in compensatory motion. According to another aspect of the invention a set of primary cranks associated with respective support arms, preferably extending outwardly from the respective pivots on the frame, are connected by a set of primary connecting rods to provide equal, dependent rotation of all of the support arms. According to preferred embodiments: primary rotation means such as the primary cranks and connecting rods just mentioned are combined with compensatory rotation means, so that the orientation of all the guide members are similarly compensated while adjusted radially; the compensatory rotation means comprises a set of secondary crank arms each associated with a respective guide member with secondary connecting rods connecting the secondary cranks to the frame, preferably the secondary connecting rods extending substantially parallel to the support arms to pivots on the frame; the frame comprises a single rigid structural ring suspended from adjustable-height cables in surrounding relation to the axis of the tube; a plurality of tiers of guide members are disposed axially along the axis of the tube, sets of axially spaced guide members being joined on a common shaft, each shaft positioned relative to the tube axis by the support arm and relative to the respective support arm by the secondary rotation means; preferably the shafts extend vertically downward as cantilevers from ends of respective support arms so that there is open space in the lower regions enabling easy access to the die, and the shafts are adjustable to position guide members at differing levels along the shaft at differing distances from the tube axis.
In the preferred embodiment the guide members are curved in the direction to approximate the curvature of the circle with overlapping relation enabling a wide range of adjustment with proper orientation of the curved members and good controlling contact with the tube.